I've heard the Pro 500 owned the Pro 900's and want to hear the signature DJ's but there is no way the bass on the D5000's wouldn't be enough. Its deeper than most but still clean and it responds well to being eq'd. I've only not heard the SigDj's but from the list there compared to the D5k's none of them can compete on a total scale. Biggest issue you'll have with any of the older Denon line is that they do not isolate very well at all. Its both a good and bad thing. There is virtually zero clamp or discomfort compared to like the M-Audio Q40's which hit like a ton of bricks but also squeeze your brain unlike any other headphone I've ever tried. The grip is of legend!! So its really personal preference. I don't mind a little sound leak because there is no way I'll be traveling with these headphones anyway so for me they're great and fit my needs perfectly.
Well at least until I save up for the D7000's